The Global Positioning System (GPS) is a global navigation system made up of a constellation of orbiting navigation satellites receivers. A GPS receiver uses signals received from three or more of the GPS satellites to determine navigational data, such as position and velocity. The GPS receiver is able to determine position and speed information by computing the relative times of arrival of the GPS signals that are simultaneously transmitted from a multiplicity of GPS satellites.
Each satellite transmits its navigation message with at least two distinct spread spectrum codes: a publicly available Coarse/Acquisition (C/A) code, and a Precise (P) code, which is usually encrypted and reserved for military applications. The C/A code is a 1,023 chip pseudo-random code having a repetition period of 1 millisecond. Each satellite transmits its own C/A code on a GPS carrier band L1 (1.57542 GHz), so that it can be uniquely identified and received separately from the other satellites transmitting on the same frequency.
The presence of a GPS signal is detected by autocorrelation of the C/A code of the received GPS signal with the locally-generated or “local” C/A code in the GPS receiver. If the received GPS C/A code is present and punctual with the local C/A code, the autocorrelation result will be at the triangular peak as shown on FIG. 1. However, if the received GPS signal C/A code is not punctual with the local C/A code, or if the received GPS signal strength is weak and/or the random noise riding on the GPS signal is dominant, such triangular shape won't be observed.
One way to improve GPS receiver sensitivity is to double the autocorrelation period. Such a technique will improve the autocorrelation quality by 3 dB, as the correlated signal gains 3 dB in power while the uncorrelated noise does not. This lengthening autocorrelation process to increase the GPS receiver sensitivity is commonly adopted in most GPS receivers, but it requires the local oscillator to be in synchronization with the received GPS carrier frequency for the best result.
Another method used to enhance receiver sensitivity of the GPS signal is called AGPS (Assisted GPS) that gets assistance from another communication link, typically a cellular network, to acquire the GPS signal. This method provides increased sensitivity to address operation indoors and in urban canyons, but demands expensive propositions for the cellular operators, handset manufacturers and GPS users. In particular, cellular operators are typically required to upgrade the existing network to support AGPS, handset manufacturers must add a GPS device in limited space, and the GPS users must pay the usage for the expensive cellular bands.
What is therefore needed is an alternative technique and system which provides increased GPS signal sensitivity and which does not suffer from the aforementioned disadvantages.